Parking Garage Supported on Re-Used Caissons and Composite Caisson Micropiles

The most sustainable foundation is the one that is already in place. That doesn’t mean it is the easiest to RE-use. This was realized when replacing a multi‑story hospital parking garage in Hartford, Connecticut. The previous garage was constructed in the 1980s and was supported on grade beams with 3.0‑ and 4.5‑foot‑diameter drilled shafts that extended through 100+ feet of problematic soft Connecticut Valley varved clay, glacial till, and end‑bearing in or on arkose bedrock. The proposed garage consisted of relatively high loading demands, was susceptible to excessive settlements, and was also located adjacent to an active hospital with sensitivity to vibrations, thereby limiting many foundation design options. An innovative solution was proposed to re-use many of the existing drilled shafts, supplemented with a combination of micropiles and composite piles. Composite piles were comprised of 3.0‑foot‑diameter drilled shafts with a central, 11.75‑inch‑diameter micropile to transfer structural loads to bedrock. Installation information on the existing drilled shafts was heavily limited. Dynamic load tests were therefore performed on 12 existing shafts spread across the garage footprint, confirming geotechnical resistances up to 4,500 kips and significantly improving quality assurance of the existing foundation construction. Additionally, two micropile load tests were performed in tension to 900 kips, confirming design loads could be resisted by the bond strength in rock. The foundation re‑use methodology saved over 3 million dollars in installation costs, weeks of construction schedule, and resulted in a significant carbon reduction compared to conventional, and inherently more substantial, foundation replacement strategies. Critical aspects of the foundation re‑use design strategy and limitations of alternatives will be discussed. Implementation of innovative load testing techniques are addressed, focusing on optimizing quality assurance for utilizing existing deep foundations in modern construction. Lessons learned and recommendations for future practice are presented.